1. Field of the Invention
The present invention relates to a charging apparatus, a process cartridge, and an image forming apparatus.
2. Description of the Related Art
In this specification, an image forming apparatus represents an apparatus which forms an image on a recording medium using an electrophotographic image forming method. The electrophotographic image forming apparatus can be, for example, an electrophotographic copying machine, an electrophotographic printer (for example, a laser printer and LED printer), a facsimile machine, a word processor, and a complex machine thereof (a multi-function printer, for example).
The process cartridge represents a member, which includes a charging device, a developing device, and a cleaning device integrally combined with an electrophotographic photoreceptor into a cartridge body. The process cartridge is capable of being attached to and detached from a main body of the image forming apparatus. It is also capable of attaching to and detaching from the main body of the image forming apparatus by integrating at least one of the charging device, the developing device, and the cleaning device with the electrophotographic photoreceptor into a cartridge body.
In the electrophotographic image forming apparatus in the related art, a corona charger (corona discharger) has been used as a charging apparatus for uniformly charging (or for diselectrifying) an image carrying member, such as the electrophotographic photoreceptor or an electrostatic recording dielectric material, so as to provide the image carrying member with the required polarity and potential.
The corona charger has a discharging electrode, such as a wire electrode, and a shield electrode surrounding the discharging polarity. The charger is disposed with a discharging opening opposed to the image carrying member in a non-contact manner. The surface of the image carrying member is charged at a predetermined potential by exposing the surface of the image carrying member to a discharging current generated by applying a high voltage to the discharging electrode and the shield electrode.
In recent years, a contact charging apparatus having lower ozone and lower electric power in comparison with corona charging have been used.
The contact charging apparatus brings a charging member to the image carrying member to charge the surface of the image carrying member to the predetermined potential and polarity by applying a bias voltage to the charging member. The charging member can be a roller type, a fur-brush type, a magnetic brush type, and a blade type.
The contact charging apparatus can include a discharging mechanism, for charging the vicinity of a contact portion between the charging member and the image carrying member, and a direct injection charging mechanism. With the direct injection charging mechanism (also referred to as direct charging, injection charging, or electric charge injection charging), an electric charge is directly injected from the contact portion between the contact charging member and the image carrying member to charge the surface of the image carrying member. In this case, since the process does not include discharging, even when a voltage applied to the contact charging member is lower than a discharging threshold, the surface of the image carrying member can be charged to a potential corresponding to the applied voltage, and no harmful effect due to resultant products of the discharging process is generated.
As a contact charging apparatus using the direct injection charging mechanism, a structure in which a conductive resilient charging roller is used as the contact charging member and conductive fine powder (charging promoting particles) for promoting charging is interposed on a contact surface between the image carrying member and the charging roller (For example, as disclosed in U.S. Pat. No. 6,038,420), and an apparatus and structure in which a charging roller and the image carrying member rotate while maintaining a relative speed difference (for example, Japanese Patent Laid-Open No. 2001-166562, Japanese Patent Laid-Open No. 2002-123062) are known. In the charging apparatus using a charging roller which is driven in contact with the photoreceptor drum, a structure in which an axis of rotation of the charging roller and an axis of rotation of the photoreceptor drum are disposed at a predetermined crossing angle for determining the longitudinally positioning of the charging roller is known (Japanese Patent Laid-Open No. 2002-072625, corresponding U.S. Pat. No. 6,600,886 B2).